Unzip (-d) the file in your Half-Life dir. You might have a new dir called "..\Half-Life\tln".
Then start H-L and, from the "Custom game" menu, select and activate "The Long Night".
Choose "New Game" from the main menu, select your skill and enjoy it!
Note that the saved games won't overwrite the ones of Half-Life, as they are stored in a different dir, so quicksave as often as you want.
To uninstall, just delete the dir.